High vibration and other issues degraded about 8.7% of the tubes in the replacement steam generators at San Onofre and led to a leak of radioactive water in one generator, according to the manufacturer of the generators.

San Onofre was shuttered after a tube in the plant's replacement steam generator system leaked a small amount of radioactive steam on Jan. 31, 2012.

What caused the problems?

The wear was blamed on tube vibration caused by excessively dry and high-velocity steam and inadequate support structures, particularly in one of the plant's two units. Tube vibration and wear has been a problem at other plants, but the specific type of vibration at San Onofre had not been experienced in the industry.

Officials with the California Independent System Operator, which oversees the state's power grid, said Monday they expect to get through summer without blackouts even if San Onofre remains shuttered — although damaging wildfires in the months ahead could undermine that prediction. With the plant out of service, the region is more dependent on imported power, and California Independent System Operator Chief Executive Steve Berberich said in an interview several weeks ago that he was "pretty concerned" that fires could threaten transmission lines in remote areas.

A long-range future without the plant is a more complicated scenario, largely because the state is also implementing new regulations on the way coastal power plants use seawater for cooling. San Onofre will loom large in upcoming discussions on whether to retire, retrofit or repower 11 gas-fired coastal plants that supply about 11,000 megawatts of power — nearly five times what San Onofre generates. The nuclear plant, which once supplied enough power for about 1.4 million homes in Southern California, has been shuttered for 15 months because of unusual deterioration of tubes in its replacement steam generators.

How much will this cost?

[Updated at 6:38 p.m.: It remains unclear. Edison has spent more than half a billion dollars on repairs and replacement power during the plant’s outage. The company also must pay into a decommissioning fund for the plant. Edison International CEO Ted Craver said Friday that there is $2.7 billion in the fund and decommissioning is 90% funded.]

Who will pay for these costs?

It has not been determined. Some activists said they will fight if Edison asks state regulators for ratepayers to cover the costs of the San Onofre problems.

How did the design problem happen?

This remains a subject of much debate. A letter sent by an Edison executive in 2004 shows the company was concerned about the possibility of design flaws in replacement steam generators manufactured by Mitsubishi Heavy Industries that could lead to the type of "disastrous" equipment problems that ultimately led to San Onofre’s shutdown. The letter was released by Sen. Barbara Boxer (D-Calif.), who argued that the letter shows that Edison may have misled federal regulators about the extent of design changes in the new steam generators. Edison strongly denies this. In a statement, Edison said that the letter showed that the company was taking care to make sure Mitsubishi addressed potential design issues. Edison Chief Nuclear Officer Pete Dietrich said in a statement, “SCE’s own oversight of MHI’s design review complied with industry standards and best practices. SCE would never, and did not, install steam generators that it believed would impact public safety or impair reliability.”